The present invention relates to an electric heating conductor having an electrically conductive core and a high temperature resisting, electrical insulation as well as an electric protective conductor configured as braided or stranded wire filaments.
German printed patent application (DE - OS) 28 50 722 describes a coaxial arrangement of a heating conductor with a fluoro-polymer as insulation. This heating conductor is provided for heating chemically aggressive fluids. The Fluoro polymer insulation is covered with a copper wire mesh whose individual wires are nickel plated to avoid corrosion. This wire mesh serves as the electric protective conductor. Particularly, this protective conductor is designed to avoid accidents that may result from short circuits within the heating conductor. The protective conductor is covered by an outer jacket made of a synthetic (e.g. a fluoropolymer) in order to provide protection against an aggressive fluid to which the heating conductor generally may be exposed or may accidentally become exposed. Such a coaxial configuration has the advantage of a wide field and range of use and employment owing to the temperature protective and corrosive fluid protections. In addition such a conductor is flexible throughout. Moreover this conductor can be manufactured in basically an endless fashion.
It was found however that occasionally, and owing to external operating conditions, a coaxial cable when subjected to strong external pressure has its outer jacket squeezed to such an extent that the insulation of the heating conductor is locally removed or at least dislodged such that the heating conductor proper and the protective conductor make physical contact or come to be placed in such a proximity that a glow or even a spark discharge obtains. In other cases of external force application, the wires of the protective conductor may break and pierce the insulation and make contact with the heating wire proper. that may actually lead to a complete failure of the heater.
The foregoing refers to factors and aspects which must be taken into consideration when designing electric heating equipment for use in explosion proof equipment and plants bearing in mind that precautionary protection against explosions poses specific as well as general design criteria in these instances. Criteria of this nature come also into play where the surface of the insulation must be covered by a protective conductor. Design criteria for assurance in this regard are given here e.g. by the German Industrial Standards (DIN) such as VDE 0170/0171. Testing of resistance against excessive squeezing followed by insulation testing is another aspect to be considered. Unfortunately it does not help to just make the insulation and the outer jacket thicker; by so doing, the problems may just become a little less frequent, but are not entirely eliminated. Aside from this consideration, making the various layers thicker increases the effective diameter and, due to the cost of the fluoropolymer, renders the cable more expensive.